"it complains that /dev/net/tun does not exist"
You're missing TUN/TAP support in the kernel. Try "modprobe tun". If that fails, you have to get the tun module somehow, or compile TUN/TAP support into the kernel. I don't know anything about TomatoUSB, but it shouldn't be hard - after all, all user-mode VPN software requires TUN/TAP.